OD Retrofitting on a vintage Volvo:
My 122 BW35 Automatic to M40/M41OD Conversion Notes: Jun 07 R. Kwas
Terminology and Clarification: An M40 is a 4 speed manual transmission with a sealed output bearing housing followed by an output flange to which the mating flange of the front driveshaft, with its universal joint, bolts. An M41 is the version of the same 4-speed trans which has instead an open output bearing housing which allows sharing the lubricating fluid with the OD which is mounted to it by means of what Volvo calls an "Intermediary Flange", and more importantly, has an extended splined output shaft (part 30 in the right diagram following) which serves as the OD input shaft. This output shaft cannot be installed without a complete transmission disassembly! Note therefore: It is not possible to simply "bolt an OD onto an M40" because of this difference, that is why it is usually better to just get the two as a set or at least know the providence and year of the vehicle a unit came from. Both have number plates from which this information can be decoded.
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Source: GCP site
Americans want their automatics!...and
Volvo of the sixties naturally offered one to them...but because of the
sluggishness and power-robbing and non-sporty nature of the Borg Warner BW35
automatic transmissions which were fitted to Amazons, conversion to a manual
transmission is often undertaken today. The standard differential ratio for
the autos was 4.30, so most conversions are are done to the M41OD combination,
because without the OD, the higher revs and associated passenger compartment
noise are a factor which most owners like to reduce (not that a B18 or B20
wouldn't be happy running like that all day), but it does become a bit much at
prolonged highway speeds (especially with a compromised exhaust system!...see:
History of J5R). There is also an associated speedometer error of
at least (4.30/4.11= , not to mention tire circumference error). The conversion to a Volvo standard transmission of the
time is the most straightforward, because bell housings are available and little
fabrication needs to be done...converting to any other trans will certainly
involve more custom fabrication work which I have no hands-on experience
with...here, I recommend posting to the Fora out there, for specifics...there are a
few brave and industrious individuals who can help here...
Once you've decided to perform the conversion, you basically procure the manual
trans plus ancillary components...getting them while still on a donor car helps
because the car is like a checklist (it's simple to look at a component and
quickly decide if you need it, but I've also included a list below)...don't
forget the pedal works including clutch (hydraulics OR cable, and this also
determines which bell housing, since they are different for hydraulic or cable
type). Under the vehicle, the M40 crossmember can be used, but needs to be
relocated forward (by means of some custom plates which need to be made is the
simplest).
List of components required:
- Pedal pivot, plus brake and Clutch pedals from a 122 (clutch may be hydraulic
if the parts come from a 122 OR later cable type if they come from a late 122 or
140, but associated bell housing must be used...hydraulics push, cable pulls to
apply force to pressure plate and disengage clutch friction plate...and they
can't be mixed and matched...cause you can't push a rope...or...well, I'm still
trying to figure out a good analogy for not being able to suck on a pipe and get
much work done on the other end!
- Flywheel/friction plate/pressure plate (I prefer the Fichtel-Sachs type) / throw-out bearing (suitable for Fichtel-Sachs, height differences exist with Borg & Beck type...again getting everything from one donor vehicle has the distinct advantage that you are certain the components all played nice together, and will again...there are enough details to worry about when doing a conversion...installing the incorrect throwout bearing and then wondering why the &#(^$*&% clutch wont disengage when everything is together shouldn't be one of them!)
- M40/Crossmember/trans-mount from a 122, recommend six cylinder motor mount type PN 1206612 / (long) front driveshaft (with similar spline-count as rear shaft...there ARE differences).
- OR M41/OD/Crossmember/trans-mount from a 122, recommend six cylinder motor mount type, (short) front driveshaft (with similar spline-count as rear shaft).
Notes on installation: Crossmember location must be moved forward to accommodate new mounting location of shorter manual trans. This may simply be done with two 1/4" plates (about 4 X 8") which bolt to the original auto-trans mount, and bring new mounting holes, which accept the crossmember, forward.
Throttle pedal will be somewhat "crowded" by the wider transmission tunnel, but one can get used to it.
Electrical considerations: There are two electrical contact functions which are performed by the BW35 auto. The "Start-Inhibit-While-In-Gear", and the "Activate-Backing-Lights-When-In-Reverse" (Item 11. Refer to wiring diagram: 122S Wiring Diagram ). The former, and its associated relay (located in the relay cluster on the driver side inner fender) falls away with the M40/M41OD conversion (remove relay, tie connector from relay terminal 87 together with connector which was on terminal 85, insulate with sleaving, tuck into harness, and forget!), and the later, should get transferred over to be activated by the M40 or M41 (by either the aluminum can switch located on the transmission case end, which monitors the reverse actuator rod, or the lid mounted switch which does the same). If an OD is being installed as a part of the conversion, IGN power to the OD activating solenoid must be supplied to engage the OD. There are a couple of versions of this...by way of a momentary switch contact which is latched by a bystable relay, which toggles its mechanical state once for every momentary activation, (like in the headlight circuit) see also: Headlight control upgrade , OR by means of a simple toggle switch located on the dashboard which applies IGN power from Fuse 1. In BOTH cases, the wire running to the solenoid should be interrupted, and therefore enabled by, the "Fourth-Gear-Sensor" switch, also located on the M41 lid, which allows the OD to be engaged ONLY while in the top gear, sparing it excessive torque of the lower gears.
Wiring of the OD: A number of different manners of controlling the OD by way of energizing the control solenoid from fused IGNition power, were employed by the factory. These include latching toggle switches on the dashboard (with and without a relay), or a momentary non-latching switch (stalk behind steering wheel).
OD Wiring 1 shows a simple mechanically latching toggle switch on the dashboard, plus a control relay, extracted from an early 1800 wiring diagram.
OD Wiring 1
OD Wiring 2 shows the momentary switch style in the form of a stalk behind the steering wheel....the relay must also perform the latching function...a bystable latching relay, similar to the headlight control relay, with mechanical toggling function, is used.
OD Wiring 2
OD Wiring 3 shows a third wiring technique which eliminates the relays altogether, extracted from a 123GT diagram. Here the switch must control all of the solenoid current instead of just a small relay control current...not a problem with a suitable switch. The observant reader will note that in this configuration, the “hot” wire must now run to the dashboard instead of just a grounding relay control wire. Of course, the indicator power wire always runs to the dash...
OD Wiring 3
The sequence of gears and shifting from third gear to fourth/OD: In all of the control circuits above, the issue still remains, that if you are in fourth gear/OD and you shift out of fourth gear without first deactivating the OD, as soon as you shift back into fourth (and close the fourth gear enable switch) you will immediately shift back into fourth gear plus OD. To prevent this (and the RPMs falling way out of the engine powerband which comes along with it), takes some getting used to...but the first few times it happens will probably bring home the point. The dashboard indicator does help in that it is wired to indicate when the control circuit is switched ON, and not the OD. As it is wired upstream of the fourth gear enable switch, it will remain illuminated when the transmission is shifted out of fourth gear without deselecting the OD, reminding the driver that the OD selection is active and will take place immediately upon shifting into fourth. ...sooo: The OD Indicator is actually more of an indicator of the control instead of the OD itself!
Given the above, the correct sequence of downshifting from fourth/OD is therefore to first deactivate the OD (by toggle or momentary), the OD Indicator extinguishes at that time, and gearing drops to fourth (only), then shifting into third, using the clutch each time to decrease shock loads on the drivetrain.
[I do recall some attempts by individuals at electronically controlled pulsing of the latching relay to automatically drop it back to the unlatched position. I don't recall seeing any successes which were simple enough to implement and which warranted publishing. Those with working circuits are welcome to contact me if they would like me to evaluate their design and publish it here.]
Which circuit should be used? None of the above circuits are technically better than the others, so if one is retrofitting a vehicle with an OD, the driver interface (switch style), and location of the OD Indicator should first be decided upon, this in-turn dictates the rest of the hardware and circuitry required. If you place value on simplicity, the single heavy duty toggle switch on the dashboard (OD Wiring 3) is hard to beat...it certainly doesn't get any simpler then that!
OD solenoid notes: The OD solenoid is one of those British automotive electrical products with questionable reliability...because on top of the manufacturer, Lucas, which is the kiss of death in itself, there is the additional complexity, of rather than being a simple coil-making-magnetism-which-pulls-in-an-armature like a starter solenoid, designers had to take into account the fact that the OD solenoid would have to endure continuous duty, unlike the Starter Solenoid, which gets to rest (and more specifically: COOL) most of the time. The way sharp sixties designers did this is by taking advantage of the fact that it takes a lot more magnetic force to initially pull in the armature, than it does to hold it once it gets to the end position. By placing two coils in the same case, a strong "pulling coil" (of heavy wire gauge, which results in a higher current and generates a stronger magnetic field) and a weaker "holding coil" (of a smaller wire gauge which allows a lower current to flow thereby generating a weaker, but adequate for holding magnetic field), and equipping the assembly with a switch which stops the (high) current-flow to the "pulling-coil" once the armature has reached the end-stop, the solenoid uses a clever pulling-current, dropping to holding-current, strategy to prevent the solenoid from frying itself to death. An OD solenoid would never survive the long-term power dissipation of the high pulling-current, but it can run continuously with just holding-current!
As can be seen in the following graphic, both coils are actually energized when applying power to the solenoid, and as the armature mechanically opens the internal switch, the current drops to a much lower level, and this in-turn drops the heat generated to a tolerable level.
OD Solenoid inner workings. "...there's more going on than meets the ear!"
(Clouseau).
OD Failure Modes: To go along with any of the hydraulic failures which can occur within the OD, any electrical failure which keeps the solenoid from being pulled into the activated position will keep the OD from engaging. This includes the typical range of external problems, from poor connections anywhere along the current path, including the at the bystable relay, to opens at the Fourth-Gear-Sensor switch on the transmission cover to mechanical looseness of the housing (which supplies the chassis return path). The typical electrical failure mode internal to the OD solenoids is that the internal switch contact intended to break the pulling-current fails to do so...the solenoid then does its best impression of a or burrito in a micro-wave...it cooks from the inside!
...that's all for now...more when I think of it. Ron
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LINKS:
Brickboard Thread: Seek circuit info on OD control relay with self-cancelling. 200
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